Feedback amplifier circuit



Patented Apr. 8, 1952 PATENT OFFICE Henricus Adrianus t" Broos,Eindhoven, Netherlands, assigner to'Hartford National Bank and TrustCompany, Hartford, Conn., .as trustee Application July. 12, 1949, SerialNo. 104,335

.In the Netherlands Aug'ufst 7, 1948 1 invention' relates tolow-frequency amplifying circui ts,A comprising an j electric dischargetu'bertliel inputjcircuit of which includes a volufnefncontol'having amovable contact and to which; the oscillationsto be amplifiedare'supplied'g'the g air1" factor of the amplier being controlla'blel bymeans of the movable contact. u The' IU. S.l patent application ,SerialNumber 23945493l'111ed`511r1e 2.211.939, now abandoned...endl merateneazazao issuedMay 12,5194;

of' positive and neg9,t,,ve, feed-back voltage re-Spectivelysgihe'.valtages of which'are proportional tothef' voltage;generated in the outputcircultof the amplifler, are included in' the'ciruitsjin such manner that substantially no negative 'feed-backvoltage o r only a small negative feed-back voltage is eifective at thetop ofthe gain control and a high negative feed-back voltage is activeat the bott'o'rnl thereof. Such 4a. circuit', comprising a sci-'calledfloss-free negative feed-back, enables the vampliiication of theamplifier to be" controlledpver a wide range, Whilst if atananiplicationuof rzero the movable contact is 'at the. bottom 'of thegain control', theremainingvolt" age generated in'theoutput circuitofthe arrangementis substantially zero.

A similar circuit is also described in the prior patent applicationSerial Number 17,243, issued Octobe'nZ, 1951 as Patent No. 2,569,973,which furthermorecomprises frequency-dependent tone lters soproportioned that, when the movable contact of the gain control occupiesa high position, W and high-tone frequencies are amplified to a smallerextentthanthe intermediate-,

tone frequencies, whereas when the said movable contact occupies 'a lowposition, the low and high-tone frequenciesare amplified to a greaterextent` than the intermediate-tone frequencies.

The said circuits involve, the disadvantage that, 'yvhe n; the movablecontact is moved to. a..

lower position s o that thesignal generated inthe output circuit `of theamplifier is 4attenuatez'i,,the

2 stantially avoiding the saiddisadvantage-with the use of a known gaincontrol having a third iixed contact between the top and the bottom.According to the invention, the bottom of the 'gainl control isconnected to a source of high negative feed-back voltage and the top ofthe gain control is connected through an impedance having a valuesmaller than the full resistance of `the gain control, to a source ofcomparatively '"small positive feed-back voltage, the third fixedcontact of the gain control being connected through an impedance havinga valuesmaller than that of the resistance portion of the gain controllocated between the third iixed contant nand 'the bottom# to a'source of negative feedback voltage which is'of an order of magnitude equal tothat of the said source of positive feedbackvoltage and smaller than thesaid source of high-negative feed-back voltage.

The invention will now be explained more fully by reference to theembodiment shown, by way ,of example, in the accompanying drawing. Thisexample is in part identical with that shown in Fig. 1 of the priorpatent application 17,243,

the reference numerals used also being similar to those of the saidfigure.

The oscillations to be amplified, for example the lowfrequencyoscillations generated in the output circuit of a detector 2 of aradio-receiver,

lid/are supplied by way of a resistance 3 to a. gain control 4, with amovable contact 6. which is included in the input circuit of anamplifying circuit 'l. The gain of the amplifier is controllable bymeans of the movable contact 6. The output Sicir'cuitof the amplifyingtube :l includes, if desired' -with the interposition of one or moreamplifying tubes, an output transformer 8, the secondary winding I0 ofwhich, is connected to a load. for examplea loud-speaker 9, and isdupli- 'ca'.te'd at the left at footof the figure. The secondary windingI0 -has a number "of taps af-e. of 'which' 'thetap c is connected toearth so that a source of positive feed-back voltage and sources ofnegative feed-back voltages are produced between the taps c-d andbetween the taps c-b,

c-e and c-a. respectively. The top Il of the gain control 4 is connectedthrough impedances I4 to 20 to the source c-d of positive feed-backvoltage, a negative feed-back voltage being made operative at the bottomIl of the gain control 4, the construction being such that substantiallyno negative feed-back or a small negative feed-back only is operative atthe top of the gain control (loss-free feed-back) The bottom Il isconnected to the source c-e of high negative feed-back voltage. A thirdfixed contact I2 provided between the top I1 and the bottom I I of gaincontrol 4 is connected through an impedance I6-v-4Il which, within thefrequency range to be amplified has a value smaller than that of theresistance portion of the gain control measured between the fixedcontacts II and I2, to the source c-b of negative feed-back voltagewhich has, for example, a value 2 to 3 times smaller. The sources c-band c--d are either of approximately equal value or the source cb ofnegative feed-back voltage may have a value slightly larger than that ofthe source c-d of Y positive feed-back voltage.

The gain control has a resistance of 0.65'

When the contact 6 moves downwards, substan;V I tial back-coupling ofthe amplifier will takeplace already in the vicinity of the fixedcontact I2 so.

that the gain factor of the amplifier rapidly decreases, notwithstandingthat the resistance portion of the gain control 4 located between the ifixed contacts Il and I2 may be of comparatively great length. Beyondthe contact I2, the output signal is reduced to such extent that poorcon- .f

ductivity between the contact' and the resistance body of gain control 4would become manifest as highly troublesome noise, were not any residualsignal and hence noise itself suppressed by the very strongback-coupling.

The circuit according to the invention may be combined in a simplemanner with that described y in the prior patent application 17,243 inwhich the impedances between the sources c--d and c-b of positive andnegative feedback voltages, the tcp il and the third xed Contact I2 ofgain control 4, are so proportioned that, when the movable contact ofthe gain control occupies a high position, low and high-tone frequenciesare amplified to a smaller extent than intermediate-tone frequenciesWhereas when the movable contact occupies a low position, the lowandhigh-tone frequencies are amplified to a greater extent than theintermediate-tone frequencies. As described in the afore-mentionedvpatent application, for this purpose an intermediate-tone filtercomprising a condenser I3,a resistance I4, a condenser I5 and aresistance IS is included between the fixed contact I2 of gain control 4and the sources c-b and c-d of -negative and .positive feed-back voltagerespectively so that, when the movable contact 6 isv in the vicinity ofthe fixed contact I2. a strong negative feed-back of' theintermediate-tone frequencies becomes effective. i .Y

Furthermore control of the high-tones may be effected with the use of apotentiometer 25 -with movable contact 21, which is connected .by way ofa small condenser 28 to the source c-a of high negative feed-backvoltage, a feed-back voltage of these low tone frequencies beingeffective at the bottom of potentiometer 25 through a. filter 2l, 22passing the low-tone frequencies. As an alternative, .instead ofutilising the said tone control, provision may be made of the iinprovedtone control circuit according to the prior patent application SerialNumber 100,438, filed June 2l, 1949.

What I claim is:

1. An electrical circuit arrangement for amplifying an input signalvoltage having frequency values within agiven range, comprising anelectron discharge tube having an input circuit and an output circuit, apotentiometer having a top `terminal, an intermediate terminal, a bottomterminal and a movable contact, said potentiometer having a first givenresistance value between said top and bottom terminals and a secondgiven resistance value between said intermediate and bottom terminals,means to apply said input signal'voltage to said top terminal, means tointercouple said movable contact and said input circuit, meanscomprising a first impedance element having an impedance value smallerthan said first given resistance value to derive a positivefeedbackvoltage .from said output circuit and to apply said positivefeedback voltage to said top terminal, means comprising a'secondimpedance element having an impedance value smaller than said secondgiven resistance value to derive from said voutput'circtu't a rstnegative feedback voltage' havingan amplitude of the same order ofmagnitudeas the amplitude of said positive feedback voltage and -toapply said rst negative 'feedback voltage to Asaid intermediate`terminal,

and means tofderive fromsaid output circuit a second negative feedbackvoltage having an amplitude value relativelyv high with respect to the.amplitudevalue o f said positive feedback voltage and Ato applysaidsecondnegative feedback voltage to vsaid bottom terminal.

2,An electrical circuit arrangement for amplifying an -input signalvoltage having frequency values -within a given range, comprising anelec- Htron discharge tube having an input circuit and anoutput'circuit, a potentiometer having a top terminal, an intermediateterminal, a bottom terminal and a movable contact, saidpotentiometerjhaving a rst given resistance value between said top andbottom terminals and a second given resistance value between saidintermediate and bottom terminals, means to apply said input signalvoltage to said top terminal, means to intercouple said movable contactand said input circuit, means comprising a rst resistance element havinga resistance value smaller than said first given resistance value toderive a positive feedback voltage from said output circuit and to applysaid positive feedback voltage to said top terminal, means comprising asecond resistance element having a resistance value smaller than saidsecond given resistance value to deriveV from s'aid output circuit afirst negative feedback voltage hav` g an amplitude substantially equalto the amp' tude of said positive feedback :voltage and to apply saidrstnegative feedback-voltage to said'intermediate terminal, and means toderive from said output circuit a second negative feedback voltagehaving an am- 1 plitude value relatively high with respect totheamplitudvalue of said positive feedback voltage and to apply said secondnegative feedback voltage to s'aidlbottom'terminal.

3. electrical circuit arrangement for amplifying van input signalvoltage having frequency values within a given range, comprising an electron discharge tube having an input circuit and an output circuit, apotentiometer having a top terminal, an intermediate terminal, a bottomvterminal and a movable contact, said potentiom- 5 eter having a. rstgiven resistance value between said top and bottom terminals and asecond given resistance value between said intermediate and bottomterminals, means to apply said input signal voltage to said topterminal, means to intercouple said movable contact and said inputcircuit, means comprising a. first impedance element having an impedancevalue smaller than said rst given resistance value to derive a positivefeedback voltage from said output circuit and to apply said positivefeedback voltage to said top terminal, means comprising a secondimpedance element having an impedance value smaller than said secondgiven resistance value to derive from said output circuit a rst negativefeedback voltage having an amplitude of the same order of magnitude asthe amplitude of said positive feedback voltage and to apply said firstnegative feedback voltage to said intermediate terminal, means to derivefrom said output circuit a second negative feedback voltage having anamplitude value vrelatively high with respect to the amplitude value ofsaid positive feedback voltage and to apply said second negativefeedback voltage to said bottom terminal, and lter means intercouplingsaid output circuit and said potentiometer to produce along saidpotentiometer a negative feedback potential having at medium frequenciesof said range an amplitude progressively greater toward said bottomterminal.

HENRICU S ADRIANUS BROOS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,173,427 Scott Sept. 19, 19392,317,025 Bond Apr. 20, 1943 2,319,663 Crowley May 18, 1943 2,367,110Fayers Jan. 9, 1945 2,374,071 Barton Apr. 17, 1945

